acct_on(): don't mess with freeze protection

commit 9419a319 upstream.

What happens there is that we are replacing file->path.mnt of
a file we'd just opened with a clone and we need the write
count contribution to be transferred from original mount to
new one.  That's it.  We do *NOT* want any kind of freeze
protection for the duration of switchover.

IOW, we should just use __mnt_{want,drop}_write() for that
switchover; no need to bother with mnt_{want,drop}_write()
there.
Tested-by: Amir Goldstein <amir73il@gmail.com>
Reported-by: syzbot+2a73a6ea9507b7112141@syzkaller.appspotmail.com
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/internal.h

@@ -89,9 +89,7 @@ extern int sb_prepare_remount_readonly(struct super_block *);
 
 extern void __init mnt_init(void);
 
-extern int __mnt_want_write(struct vfsmount *);
 extern int __mnt_want_write_file(struct file *);
-extern void __mnt_drop_write(struct vfsmount *);
 extern void __mnt_drop_write_file(struct file *);
 
 /*

include/linux/mount.h

@@ -87,6 +87,8 @@ extern bool mnt_may_suid(struct vfsmount *mnt);
 
 struct path;
 extern struct vfsmount *clone_private_mount(const struct path *path);
+extern int __mnt_want_write(struct vfsmount *);
+extern void __mnt_drop_write(struct vfsmount *);
 
 struct file_system_type;
 extern struct vfsmount *fc_mount(struct fs_context *fc);

kernel/acct.c

@@ -227,7 +227,7 @@ static int acct_on(struct filename *pathname)
 		filp_close(file, NULL);
 		return PTR_ERR(internal);
 	}
-	err = mnt_want_write(internal);
+	err = __mnt_want_write(internal);
 	if (err) {
 		mntput(internal);
 		kfree(acct);
@@ -252,7 +252,7 @@ static int acct_on(struct filename *pathname)
 	old = xchg(&ns->bacct, &acct->pin);
 	mutex_unlock(&acct->lock);
 	pin_kill(old);
-	mnt_drop_write(mnt);
+	__mnt_drop_write(mnt);
 	mntput(mnt);
 	return 0;
 }